Heat-sensitive sheet

ABSTRACT

A HEAT-SENSITIVE SHEET USING A MIXTURE OF P-CRESYL PHOSPHATE AND OTHER ORGANIC PSHOSPHORIC ACID ESTERS SUCH AS TRIPHENYL PHOSPHATE, TRIBUTYL PHOSPHATE, 2 - ETHYLHEXYL PHOSPHATE AND MIXTURES OF TRICRESYL PHOSPHATE ISOMERS.

United States Patent O 3,674,538 HEAT-SENSITIVE SHEET Takashi Suzuki,Ikeda-shi, Japan, assignor to Matsushita Electric Industrial Co. Ltd.,Osaka, Japan No Drawing. Continuation-impart of application Ser. No.

514,800, Dec. 20, 1965. This application Apr. 8, 1969,

Ser. No. 814,433

Int. Cl. 341m 5/26 US. Cl. 11736.7 6 Claims ABSTRACT OF THE DISCLOSURE Aheat-sensitive sheet using a mixture of p-cresyl phosphate and otherorganic phosphoric acid esters such as triphenyl phosphate, tributylphosphate, 2 ethylhexyl phosphate and mixtures of tricresyl phosphateisomers.

CROSS-REFERENCES TO RELATED APPLICATION This is a continuation-in-partof application Ser. No. 514,800 filed on Dec. 20, 1965, and nowabandoned.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a heat-sensitive sheet for reproduction using thesupercooling property.

The method of thermographic reproduction using supercoolable liquid andpowder developer has been publicly known. According to this method, theheat-sensitive material of a copying sheet is made of a substance whichis solid at room temperature but has good supercooling property whenmelted and liquefied. The copying sheet is superposed on an original,and intense infrared ray is irradiated thereon to raise the temperatureon the image portion of the original and melt the heat-sensitivematerial. This melting state still remains after the removal of the heatsource. The powder developer introduced to this state adheres to thesurface of the melting liquid whereby an image is obtained.

This invention is related to such a heat-sensitive copying sheet as isused in the above-method.

Description of the prior art Inorganic substances having goodsupercooling property heretofore obtained are sulphur, antimony, sodiumhyposulfite, and sodium acetate, while organic ones are acetanilide,phenacetin, benzotriazole, acetylthiocarbacid, dimedonebenzoic acid,sebacic acid, citirc acid, cinnamic acid and their suitable mixtures.

The important factors are that these materials for reproduction arechemically stable, stably supercooled, colorless and odorless, harmlessto the human body, have a melting point within a suitable temperaturerange, are free from decomposition even if heated considerably higherthan the melting point, selectively dissolved in solvent, easilypulverized, and easily manufactured.

According to the inventors experiments, the above publicly knownsupercoolable substances do not fully satisfy these necessaryconditions. So, they are not always suitable as heat-sensitive materialfor reproduction.

For example, hyposulfite and sodium acetate lose a part ofcrystallization water at a temperature slightly higher than the meltingpoint and change into other substances which have no more supercoolingproperty. The manufacture of heat-sensitive sheet is easily done bydiluting the heat-sensitive substance in solvent and impregnating it ina porous base. But the mixture of benzotriazole with acetanilide or withphenacetin is not suited to such a dissolution method in spite of theirgood supercooling property. The reason is not yet clear but lies perhapsin the fact that two substances are separated from each other when thesolvent is evaporated.

The inventor has found that triphenyl phosphate is a very excellentheat-service material for reproduction. Although triphenyl phosphatesatisfies almost all of the above-mentioned necessary conditions, a weakpoint thereof is that the melting point is low, i.e. 49 C. So, when alarge pressure is applied to the sheets stacked for preservation, theyare softened so that the possibility arises that they will adhere toeach other. Another weak point is that the heat-sensitive temperaturediifers only a little from the room temperature. Unless the infraredsource is very strong and uniform, it is diflicult to copydistinguishably the thin and thick lines drawn in the original.

SUMMARY OF THE INVENTION Therefore, the purpose of this invention is toimprove such shortcomings as mentioned above and to provide aheat-sensitive material for reproduction with a melting point between 65and C. satisfying the aforementioned conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventor has found that themixture of p-tricresyl phosphate with other phosphoric esters is verysuitable for the above-mentioned objects.

p-tricresyl phosphate is a colorless, incombustible and stable substancehaving a melting point in the range from 77 to 78 C. But itssupercooling property is not satisfactory for the copying heat-sensitivematerial. However, if other phosphoric esters are added, thesupercoolness is found to be remarkably improved, yielding a verysuitable heat-sensitive material. The esters are triphenyl phosphate,tricresyl phosphate mixed with isomers that are customarily used asplasticizer, tributyl phosphate, and 2-ethylhexylphosphate and the like.When the amount of such additives is increased the supercooling propertyis promoted but the melting point is decreased. This tendency is similarwith all adding agents but the degree differs with the kind of esters.

With a 40% addition of triphenyl phosphate the melt ing point becomes 69C. while with a 40% addition of tricresyl phosphate mixed with isomersit becomes about 60 C. Tricresyl mixed isomers are, of course, a mixtureof o-tricresyl phosphate, m-tricresyl phosphate and p-tricresylphosphate.

When the formation of the sheet is performed by diluting theheat-sensitive substance in solvent and im pregnating it in a porousbase, a 10% addition of the above-mentioned esters yields the bestsupercooling property. When the heat-sensitive material is obtained bybeing dispersed in aqueous solutions of polyvinyl alcohol, gelatine andthe like and thereafter coated on a base, a 1 to 20% addition of themyields good results. Similar results are obtained with other kinds ofphosphoric ester.

From the above results the most preferable composition of theheat-sensitive material is 99 to 65% of p-tricresyl phosphate and 1 to35% of other phosphoric esters.

However, the above-mentioned composition does not always remain the bestfor other manufacturing methods and fabrication conditions. Thecomposition is not restrictive.

Next, the embodiments will be described.

EXAMPLE 1 p-Tricresyl phosphate 1.6 g. triphenyl phosphate 0.4 g. weredissolved in toluene 1000 cc. A sheet of frosted tracing paper wasdipped in the above solution, pulled out and dried at room temperature.The sheet was superposed on an original, which was a sheet of highquality paper printed with black ink. A 375 w. infrared lamp wasirradiated for about 3 seconds from 2 cm. above the sheet. Theheat-sensitive sheet was developed by powder developer used inelectrophotography. As a result, a good copy was obtained.

EXAMPLE 2 pTricresyl phosphate 8.8 g. and tricresyl phosphate mixedisomers 1 g. were heated at 90 C. to 110 C. for 10 minutes and flowedinto aqueous solution of polyvinyl alcohol 100 cc. They were mixed anddispersed by a juice mixer, and coated to 50 thick on a triacetylcellulose film of 0.05 mm. thickness. The heatsensitive sheet wasexamined under the same condition as in embodiment 1. About 2 minutesirradiation of an infrared lamp yielded a copy.

As is evident from the foregoing description, this invention relates toa heat-sensitive sheet using a heat-sensitive substance made of amixture of p-tri'cresyl phosphate and other kinds of phosphoric ester.By a suitable selection of their composition ratio the heat-sensitivesheet can have the best thermographic temperature, supercooling propertyand stability.

1 claim:

1. A sheet for thermographic reproduction having a heat sensitivecoating thereon which is normally crystalline at room temperature butsuper cools and remains liquid after it has been melted comprising asthe material which super cools a mixture of p-tricresyl phosphate andother organic phosphoric acid esters selected from the group consistingof triphenyl phosphate, tributyl phosphate, Z-ethylhexyl phosphate and amixture of tricresyl phosphate isomers.

2. A sheet according to claim 1 wherein said organic phosphoric acidesters are selected from the group consisting of tripheuyl phosphate anda mixture of tricresyl phosphate isomers.

4 3. The sheet of claim 1 wherein the p-tricresyl phosphate is mixedwith triphenylphosphate.

4. The sheet of claim 1 wherein said mixture contains a mixture oftricresyl phosphate isomers.

5. The sheet of claim 1 wherein the mixture contains from about to about99% p-tricresyl phosphate and from about 1% to about 35% of said otherorganic phosphoric acid esters.

6. The sheet of claim 1 wherein the heat sensitive coating has a meltingpoint between 65 and 100 C.

References Cited OTHER REFERENCES Noller, 1957, page 515.

WILLIAM D. MARTIN, Primary Examiner B. D. PIANALTO, Assistant ExaminerUS. Cl. X.R.

117-17.5, 25,122 H, 154, 161UE

